The discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor and, moreover, any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.
The ‘greenhouse effect’ and ongoing atmospheric pollution are significant ecological problems. The main gasses responsible are water vapour, carbon dioxide, methane, nitrous oxide and ozone. The relative contributions of these gasses to atmospheric pollution and the greenhouse effect depend on the characteristics of each gas and its abundance. For example, methane has characteristics that make it significantly more potent than carbon dioxide as a greenhouse gas but carbon dioxide has a greater contribution based on its quantity. The growth of industry and the burning of fossil fuels since the industrial revolution have substantially increased the levels of carbon dioxide in the atmosphere.
Various schemes have been mooted for reduction in greenhouse gas emissions. Many economists believe that putting a price on carbon is an essential starting point—that is, putting a price on carbon so that there is an incentive for people to stop emitting greenhouse gasses.
Large scale removal of carbon dioxide from industrial sources to avoid atmospheric emission is an ongoing problem. Processes for acid gas removal are well known and used widely. However, it is costly to achieve significant reduction of industrial carbon dioxide emissions, and improving the cost effectiveness is an ongoing challenge. Processes for carbon dioxide removal have an impact on the cost of downstream goods/services. Accordingly, the process cost must be balanced against this impact if the process is to be acceptable to the relevant industry. In a carbon constrained world, all industries are exposed to carbon dioxide emission costs, irrespective of which process (if any) they choose. Processes or systems that drive down the costs of carbon dioxide removal either through improved technological solutions, lower life cycle costs or reduced supply chain impacts are likely to be preferred. Those who develop such processes or systems at an early stage of the technology may concomitantly be able to take advantage of the opportunity to earn early benefits.
For example, some industries are adopting a new process for avoiding carbon dioxide emission to the atmosphere by capture, concentration and storage of the carbon dioxide in deep geological structures. This is known as carbon capture and storage (CCS). The capture stage of CCS removes carbon dioxide from various fossil fuel burning sources and three alternative approaches form the basis of the majority of research,                Post Combustion (PCC) which takes low pressure gas from conventional fossil fuel burning sources and removes pure carbon dioxide        Pre-Combustion which removes carbon dioxide from high pressure sources such as synthesis gas prior to complete combustion for power and/or further product synthesis and,        Oxyfuels where air is replaced by oxygen for combustion of fossil fuels thereby simplifying carbon dioxide separation.        
The cost benefit varies from industry to industry. For example, the electricity production industry will assess the use of CCS systems based on the cost of electricity generation and the commercial impact in the relevant power markets.
On a purely commercial assessment (setting aside early stage transitional development phases and the incentives that may be available) CCS is likely to only be acceptable from the point in time when overall technology costs intersect with carbon dioxide prices (see FIG. 5). Processes or systems that drive down the costs of CCS and the resulting impacts on products, such as the cost of power as measured by the levelised cost of electricity (LCOE), either through improved technological solutions, lower life cycle costs or reduced supply chain impacts are likely to be preferred and help accelerate building of a large scale CCS industry. This will provide concomitant opportunities to owners of such technologies to earn early benefits.
Some CCS applications provide by-product or service benefits. These include the use of carbon dioxide for enhanced oil recovery (EOR) or the production of liquid fuels from synthesis gas. The latter has been successfully used for production of liquid fuels from coal gasification with CCS. The inclusion of a revenue stream rather than sole reliance on carbon pricing to justify investment provides motivation for early adoption of CCS.
Nitrogen compounds (mainly amines and ammonia) have been a focus for research into carbon dioxide capture processes. The use of alkali carbonate processes has been less actively pursued. Even less interest has been shown in identifying the fate of impurities such as sulphur and nitrogen and optimising their downstream uses other than through the addition of flue gas desulphurisation and nitrogen removal equipment to limit consumption of, and adverse reactions with solvents. The proponents of the chilled ammonia process refer to the production of ammonium sulphate as a fertiliser by product. Recently concerns about the fate of nitrogen based degradation products such as nitrosamines has created increased research into amine based solvents in PCC and concerns regarding their fate.
Most activity relating to reduction in the overall cost of carbon capture has been directed to either consideration of the process itself or the product/service opportunities described above. Historically amines have represented the most energy and cost efficient target for emission systems already fitted with impurities handling units such as flue gas desulphurization (FGD) units.
Accordingly, there has been a disproportionate amount of research directed to amine based capture routes which only produce waste products.
Comparatively little attention has been paid to other processes for carbon capture. These waste streams would have significant impact on the makeup rates and supply chains for the base solvent. In the case of amine the rates of consumption (calculated as the product of the specific losses of solvent, measured in kilograms solvent per tonne of carbon dioxide, and the large quantities of carbon dioxide for capture) will require significant additional capacity in global amine chemicals production. This requirement for additional feedstock supply resulting in the disposal of a waste product would continue to be a logistical and economic burden carried by the technology.
However emerging carbonate options can reduce the energy penalty for carbon dioxide removal and also allow combined removal of carbon dioxide with other impurities. For example, some current processes remove carbon dioxide from industrial emissions by passing the gas through aqueous potassium carbonate solution circulating through an absorption column (sometimes referred to as a scrubber) (see FIG. 4). The basis of this process is (1) hydration of carbon dioxide in a reversible reaction to form carbonic acid, which in turn reacts with a carbonate ion to form two bicarbonate ions (2) (potassium provides the cation in this case though other ions could be used)CO2+H2OH2CO3  eqn (1)H2CO3+CO32−2HCO3−  eqn (2)
The process is completed by processing the bicarbonate laden solvent stream to regenerate the carbonate (generally through the application of heat) in a regenerator (sometimes referred to as a stripper) and releasing the carbon dioxide as a purified stream. This process allows the solvent to be recirculated continually for further carbon dioxide removal in a closed loop system with the only makeup being for system losses.
Carbonate absorption/stripping systems like this can be operated in various modes such as PCC, pre-combustion or indeed any application where CO2 is to be removed.
In most solvent processes, particularly with amines which are highly susceptible to attack by other acid gases such as oxides of sulphur, the gas is pre-treated to remove impurities to low levels otherwise the losses of solvent would make the process un commercial.
However in the case of potassium carbonate the reactions of these impurities with the solvent can produce potentially useable by-products. The end products would be potassium sulphate and potassium nitrate which could be reused back In the fertiliser industry from whence the base potassium came. It should be noted the single most important commercial use of potassium products is for fertiliser. The agricultural sector is constantly looking for sources of nitrogen, phosphorus and potassium (commonly referred to as NPK). The broad reactions of these the gas impurities with potassium, using SO2 and NO2 as examples are:2K2CO3+2SO2+O2→2K2SO4+2CO2 2K2CO3+4NO2+O2→4KNO3+2CO2 
While this example indicates the reactions in an oxidising environment similar reactions can be described for other capture circumstances such as found in syngas or pre-combustion capture applications.
Furthermore, other than for CCS incorporating enhanced oil recovery or returns from syngas fuels, effectively all commercial improvements in CCS, particularly in PCC, focus on cost reductions due to either solvent performance or configurations and heat integration with the power plant leading to reduced variable and/or equipment cost reduction.
There is therefore a need for novel additions to, and configurations of, carbon capture that further improve the life cycle impact and commercial attractiveness of low emission technologies, and particularly when operated in a post combustion mode.
One approach to producing higher value products from carbon dioxide removal has been described in International patent applications WO 2006/034339 and WO 2009/039445. These patent applications teach the use of sodium hydroxide scrubbing on a ‘once-through’ basis to produce carbonate and bicarbonate products. Significant modifications to electrolysis and scrubbing processes are taught to achieve what is described as ecological efficient removal of carbon dioxide. This process produces a carbonate/bicarbonate product which can be considered either as a by-product or a mineral based method for permanently sequestering carbon dioxide. This differentiates it from other geological methods of carbon dioxide sequestration used for CCS. The prior art patents disclose transportation of the carbonate products to CCS sites, along with chemicals which may be used to generate carbon dioxide for geological storage. However this increases the complexity of the CCS chain.
Given the very large quantities of carbon dioxide emitted from a power station (and the potential need for at least about 90% carbon dioxide removal) the ‘once-through’ nature of this process creates two problems, namely the internal use of electricity and the large volume of carbonate and other products.
The conventional electrolysis process used to produce the necessary hydroxide for complete conversion of carbon dioxide to carbonate products is in excess of the power available from the power station. For example, FIG. 9F of International patent application WO 2006/034339 indicates that the electrolysis needs exceed the generation of power by 12%. Should that situation be maintained the carbon dioxide removal process (for that purpose alone) would be of little use with no power being available for sale by the generator. WO 2006/034339 teaches a number of modifications and integrations which are necessary for use in the process to recover the heat and power and use them internally to reduce the overall power requirement by the electrolyser.
Furthermore the quantities of product produced from such a process are likely to compromise its usefulness due to the flooding of chemical markets with one or all of the by-products. For example, International application WO 2006/034339 includes exemplification based on a single 1000 MW power station. FIG. 9C of WO 2006/034339 indicates that the combined total carbon dioxide and sodium hydroxide produced by the example, which together approximate the sodium bicarbonate production rate, are over 15 million tonnes per annum. This is in excess of the nameplate capacity of the production of all soda ash producers in the United States in 2003.
Similarly, the chlorine production referred to in FIG. 9D is approximately 6 million tonnes per annum. This may be five to ten times the size of the largest chlorine plants in the world.
Accordingly there is a need for processes and systems for large scale carbon capture and geological storage that provides improved overall cost attractiveness to end users by producing additional useable products.